import React, { Component } from 'react';
import ReactDOM from 'react-dom';
 

class Clock extends Component{
    constructor(props){
        super(props);
        this.state={date:new Date()};
    }

    render(){
        return (
            <div>
                <h1>Hello, world!</h1>
                <h2>It is {this.state.date.toLocaleTimeString()}.</h2>
            </div>
        );
    }
    componentDidMount() {
        this.timerID = setInterval(
            () => this.tick(),
            1000
          );
    }
    componentWillUnmount() {
        clearInterval(this.timerID);
    }
    tick() {
        this.setState({
            date: new Date()
        });
    }
}

ReactDOM.render(
    <Clock  />,
    document.getElementById('root')
);
 